Something doesn't seem right about your tranny, no stock valve body should be able to engage low gear form second or high gear above 4500 RPM or above something is goofy
I have done similar, shifted 3rd to reverse at the end of a 1/4 ,mile trying to shift it into neutral in one of the old NHRA hemi stocker, it went into neutral and then started to rev up engaging reverse making the rear end almost lock up, I pulled the stock comsole shifter back out of reverse and managed to get it into 2nd and then shoved it back into high gear, what a cluster frick that was